If you are receiving the 'Add-in Error' message, follow these steps to reinstall Yesware for Office 365:

In your O365 Inbox:

1. Click 'Get Add-ins' > My add-ins > Yesware - click the 3 dots and select 'Remove'
2. Close Outlook
3. Re-open Outlook
4. Click 'Get Add-ins' > search for Yesware and select 'Add'

Next, within Outlook, please go to "File" > "Office Account" and click on "Office Updates" and in the dropdown select "Update Now". Then close out of Outlook completely and open it back up.

Then, open the Yesware Sidebar again and see if you are still receiving the "Add-In Error" message.

Please make sure to also have the Yesware Sidebar pinned open, within the New/Compose email window.  

Please reach out to Yesware Support if this does not resolve the issue.